I don't know if you heard of Zabuton. A zabuton is a sitting "futon" or sitting mat. It is used to sit on. :) In Japan when I was growing up, it was pretty common to sit on the floor and we used zabuton all the time.
I know people in the west don't usually sit on the floor much, but sometimes it is still comfortable sitting on the floor. Therefore, I had to make some zabuton myself.
I researched the size for them, and they are usually about 50 cm x 50 cm. I am sure you can make it smaller or larger as you like. I upcycled an old Japanese futon to to use in these zabutons and other bench seats. The old wool mattress has gotten quite flat to use as a mattress, but it was still good for these uses.